(1) Field of the Invention
The present invention relates to a pouring nozzle for the transfer of molten metal from an upper metallurgical vessel to a lower metallurgical vessel. In particular, it concerns a pouring nozzle of refractory material for the transfer of molten steel from a tundish to an ingot mold or, alternatively, from a casting ladle to a tundish.
(2) Description of the Related Art
The pouring nozzles intended for transferring molten metal from a metallurgical vessel to another while protecting the metal against chemical attacks and isolating it thermally from the surrounding atmosphere are wear elements which are strongly stressed to an extent that their service life can limit the casting time. Devices for the nozzle insertion and/or removal recently described in the state of the art have permitted to solve this problem (see for example European patents 192,019 and 441,927). For example, as soon as the nozzle external wall erosion at the vicinity of the meniscus reaches a certain level, the worn nozzle is exchanged with a new nozzle in a period of time sufficiently short for not having to interrupt the casting.
Generally in these devices, one will use a pouring nozzle constituted of a tubular part defining a pouring channel and, at its upper end, of a plate provided with an orifice defining a pouring channel, said plate comprising an upper surface contacting the upstream element of the pouring channel and a lower surface forming the interface with the lower part of the nozzle, said lower surface comprising two planar bearing surfaces located on both sides of the pouring channel.
The nozzle is intended to slide in guides against the planar lower surface either of a pouring orifice such as an inner nozzle, of a bottom plate affixed to such a pouring orifice or of a fixed plate affixed to a casting flow control device inserted between the pouring orifice (inner nozzle for example) and the pouring nozzle. It must be clear that in the context of the present invention, when reference is made to a pouring nozzle, this nozzle intended to slide in a device and is not a fixed nozzle such as an inner nozzle.
Known devices and particularly the device disclosed in the document EP 192,019, have a pouring nozzle sliding into guides able to transmit a thrust force upwardly (pushing device). This thrust force is obtained by springs arranged at a certain distance of the pouring orifice and actuating levers or rockers. These transmit the thrust force to the planar surfaces of the pouring nozzle plate. This upwardly directed thrust force pushes relatively tightly the pouring nozzle plate against the upstream refractory element, notably an inner nozzle or a refractory plate.
Pouring nozzles can be mono-block or can be constituted of an assembly of several refractory elements.
In most of the cases, the lower surface of the plate and the upper end of the tubular part of the nozzle are protected by a metallic can.
It has however often been noted that cracks or micro-cracks can appear at the level of the junction between the tubular element and the plate, located at the upper end of the tubular element. These cracks can occur when the nozzle is serviced or during its use. The origin of the cracking can be an excess of thermal stresses, of mechanical stresses or of thermo-mechanical stresses. These stresses are generated by the forces exerted to maintain the nozzle in the device, by vibrations and by the liquid metal flow.
In certain cases, these cracks induce the rupture of the element. In other cases, even though these cracks have a tiny size, it is necessary to take them into account. The throttling generated by the flow of liquid metal in the nozzle creates indeed a low pressure and, consequently, induces an important aspiration of the ambient air. The atmospheric oxygen and even nitrogen are important contamination sources for the liquid metal, in particular of steel. Further, under the combined action of the oxygen and of the very high temperatures, the refractory material can considerably deteriorate at the oxygen entry level, i.e. at the crack level. This deterioration increases the local deterioration of the refractory material and widens the crack to such an extent that it can be necessary to stop the casting.
There are several means provided in the state of the art to increase the resistance of the nozzle against cracking.
Refractory materials having a better resistance to cracking are known. Nevertheless, these materials are generally sensitive to other phenomenon such as erosion or corrosion.
Another solution disclosed in the document WO 00/35614 is the use of a metallic can reinforced at its lower part by mechanical means which increase its stiffness.
The document EP 1,133,373 describes a nozzle comprising a shock-absorbent intermediate region between the metallic can and the refractory nozzle. This region is comprised of a material whose thermal properties are such that it remains solid at ambient temperatures but is subjected to deformation at high temperatures. This buffer region reduces the risks of formation of cracks or micro-cracks generated by the thermo-mechanical stresses appearing at the beginning of the casting.
Despite the advantages brought to the art by the above described solutions and their continuous improvements during these last years, there are still some problems.
Indeed, in the known devices for the nozzle insertion and/or removal, the plate is always subjected to important flexural stresses which can be responsible for the formation of cracks at the upper end of the tubular part. It has indeed been observed that the upper plate can deform by flexion around an axis parallel to the direction of the guides where the said plate slides.
The above described solutions permit to lower these flexural stresses by stopping them or by diluting them and this, by acting on the material itself or on the nozzle assembly techniques. These solutions are expensive and not fully satisfactory.